Relaxation delay time

Figure 2-13 Apparent phase errors due to signal-truncation effects, (a) The distorted spectrum caused by an acquisition time that is too short (DT = 0 s). (b) The distortion-free spectrum after the introduction of a relaxation delay time (DT = 1 s).

Because DEPT is a polarization transfer experiment, the relaxation delay times are a function of the H-, and not the X-nucleus , T s. The following are additional suggested spectral parameters ... 

Almost all of the experiments described in this chapter, and certainly all 2D experiments, require some time between scans during which the spin systems that have been irradiated or otherwise perturbed can return to equilibrium. There are two times during which relaxation processes occur (i) the relaxation delay time (DT), which is the period between the end of the acquisition of the signal (ta) and the first pulse of the pulse sequence being used and (ii) the repetition time (RT), which is the sum of DT + tg- Since relaxation occurs during as well as DT, especially for H-detected pulse sequences, it is important to consider RT, and not just DT, when deciding on experimental delay times between pulses. If, for example, it is determined that RT ought to be about 1 s and tg 200 ms, then DT should be set to approximately 800 ms. 

Like COSY, HETCOR is a relatively robust sequence. Unlike COSY, it can be performed reasonably in either the absolute-value or phase-sensitive mode, although the latter gives better resolution. Since it is relatively immune to artifacts (if pulsing is not too rapid), a gradient version is largely unnecessary. Because HETCOR is a polarization transfer experiment, the relaxation delay times are a function of the H-, and not the X-nucleus, Tj s. 

The same considerations that are relevant for the selection of mixing times and relaxation delay times for NOESY experiments also apply to ROESY [i.e., suggested RT s are about 2ri for small to intermediate-sized molecules (MW 400-750) and should generally be in the 0.9-1.8-s range (Section 7-lOa)]. Spin-lock mixing times are in the 100-500-ms range—closer to 100 ms for larger molecules and 400-500 ms for smaller molecules. 

The remarkable stability and eontrollability of NMR speetrometers penults not only the preeise aeeiimulation of FIDs over several hours, but also the aequisition of long series of speetra differing only in some stepped variable sueh as an interpulse delay. A peak at any one ehemieal shift will typieally vary in intensity as this series is traversed. All the sinusoidal eomponents of this variation with time ean then be extraeted, by Fourier transfomiation of the variations. For example, suppose that the nomial ID NMR aequisition sequenee (relaxation delay, 90° pulse, eolleet FID) is replaeed by the 2D sequenee (relaxation delay, 90° pulse, delay i -90° pulse, eolleet FID) and that x is inereased linearly from a low value to ereate the seeond dimension. The polarization transfer proeess outlined in die previous seetion will then eause the peaks of one multiplet to be modulated in intensity, at the frequeneies of any other multiplet with whieh it shares a eoupling. 

Figrue BE 16.20 shows spectra of DQ m a solution of TXlOO, a neutral surfactant, as a function of delay time. The spectra are qualitatively similar to those obtained in ethanol solution. At early delay times, the polarization is largely TM while RPM increases at later delay times. The early TM indicates that the reaction involves ZnTPPS triplets while the A/E RPM at later delay times is produced by triplet excited-state electron transfer. Calculation of relaxation times from spectral data indicates that in this case the ZnTPPS porphyrin molecules are in the micelle, although some may also be in the hydrophobic mantle of the micelle. Furtlier,... 

Fig. 7. A C-13 relaxation time measurement of solid state wetted cellulose acetate (6% by weight water) using the inversion recovery (IR) method at 50.1 MHz and spinning at 3.2 kHz at the magic angle (54.7 deg) with strong proton decoupling during the aquisition time (136.3 ms), (upper part of the Figure). Tau represents the intervals between the 180 deg (12.2 us) inverting and 90 deg (6.1 us) measuring pulse. 2200 scans were collected and the pulse delay time was 10 s, Cf. Table 3 and Ref.281...

The relaxation rates of the individual nuclei can be either measured or estimated by comparison with other related molecules. If a molecule has a very slow-relaxing proton, then it may be convenient not to adjust the delay time with reference to that proton and to tolerate the resulting inaccuracy in its intensity but adjust it according to the average relaxation rates of the other protons. In 2D spectra, where 90 pulses are often used, the delay between pulses is typically adjusted to 3T] or 4Ti (where T] is the spin-lattice relaxation time) to ensure no residual transverse magnetization from the previous pulse that could yield artifact signals. In ID proton NMR spectra, on the other hand, the tip angle 0 is usually kept at 30°-40°. 

The whole sequence of successive pulses is repeated n times, with the computer executing the pulses and adjusting automatically the values of the variable delays between the 180° and 90° pulses as well as the fixed relaxation delays between successive pulses. The intensities of the resulting signals are then plotted as a function of the pulse width. A series of stacked plots are obtained (Fig. 1.40), and the point at which the signals of any particular proton pass from negative amplitude to positive is determined. This zero transition time To will vary for different protons in a molecule. 

A number of parameters have to be chosen when recording 2D NMR spectra (a) the pulse sequence to be used, which depends on the experiment required to be conducted, (b) the pulse lengths and the delays in the pulse sequence, (c) the spectral widths SW, and SW2 to be used for Fj and Fi, (d) the number of data points or time increments that define t, and t-i, (e) the number of transients for each value of t, (f) the relaxation delay between each set of pulses that allows an equilibrium state to be reached, and (g) the number of preparatory dummy transients (DS) per FID required for the establishment of the steady state for each FID. Table 3.1 summarizes some important acquisition parameters for 2D NMR experiments. 

Pre-saturation In this technique prior to data acquisition, a highly selective low-power rf pulse irradiates the solvent signals for 0.5 to 2 s to saturate them. No irradiation should occur during the data acquisition. This method relies on the phenomenon that nuclei which have equal populations in the ground and excited states are unable to relax and do not contribute to the FID after pulse irradiation. This is an effective pulse sequence of NOESY-type pre-saturation that consists of three 900 pulses RD - 900 - tx - 900 - tm - 90° - FID, where RD is the relaxation delay and t and tm are the presaturation times. 

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